The invention relates to a premix burner for a heat generator, in particular for use in a gas turbine system according to the preamble of Claim 1.
EP 0 321 809, WO 9317279, as well as EP 0 945 677 have disclosed premix burners in which a combustion air stream is fed via a swirl generator tangentially into an interior burner chamber and is mixed with fuel. Gaseous fuels, e.g. natural gas, are injected along tangential air inlet slits into the combustion air stream, whereas liquid fuels, such as heating oil, are injected preferably via a central nozzle at the burner head. At the burner outlet, the resulting eddy current bursts open at a change in the cross-section, inducing a backflow zone used for stabilizing the flame while the burner is being operated.
In order to prevent interference with the flow field under the conditions of use in gas turbines, even if fuels of different origin and composition are used, and in this way always achieve a safe flame position, the teaching of EP 0 780 629 provides that downstream from the swirl generator a mixing section may be provided that ensures a better premixing of different types of fuels.
While burners designed in such a way enable operation with very low noxious emissions, they frequently operate very close to the extinction limit of the flame: The usual flame temperatures achievable with the lean premix flames of such burners are about 1700 K to 1750 K. Under certain operating conditions, the extinction limit of the flames may already be reached at 1650 K. This value is relatively high. The reason for this is the low fuel content of the fuel/air mixture that reduces the flame speed, which then results in a spatially larger and therefore more instable flame front.
However, a richer mixture would increase the noxious emissions and make the use of lean premix burners absurd.
It is also known that many times thermoacoustic oscillations occur in the combustion chambers of gas turbines, resulting in undesired effects, such as too high mechanical stress, increased NOx emissions due to inhomogeneous combustion, and even extinction of the flame. Since, in order to achieve low NOx emissions, an increasing part of air is passed through the burners themselves, the sound-dampening effect of the cooling air flowing into the combustion chamber is reduced, so that the problems associated with undesired oscillations increasingly occur especially in such modern gas turbines.
The present invention is based on the objective of improving the stability of the lean premix combustion of modern burners of the initially mentioned type, as used in particular in the combustion chambers of gas turbines. In order to ensure operation with low noxious emissions, any significant increase in the combustion temperature must hereby be avoided.
According to the invention, this objective is realized with a premix burner of the type mentioned in Claim 1. The secondary claims represent advantageous embodiments of such a burner.
The basic idea of the invention consists of equipping a mixing section of a premix burner with a net-like structure that extends at least almost completely over the flow cross-section of the mixing section.
According to a preferred embodiment, the net-like structure consists of a wire mesh or a plurality of layers of wire mesh arranged at a distance from each other.
An arrangement of five to 100, in particular ten to 20, wire mesh fabrics oriented at least approximately vertically to the longitudinal burner axis was hereby found to be advantageous.
According to an alternative embodiment, the net-like structure consists of a porous body, made of a foam material that is integrated into the burner chamber.
The net-like structure preferably should be designed so that it causes no negligible pressure loss or formation of local eddies that could substantially affect the flow in the mixing section.
In an embodiment based on wire mesh fabrics, this is ensured by selecting the dimensions of the wire thickness and mesh width with respect to each other in such a way that the largest possible, open sieve surface, preferably in a magnitude of more than 90%, remains.
By varying the open sieve surface across the cross-section, it is possible to influence the flow conditions in the mixing section in a targeted manner.
In this context, it was found to be advantageous to support the edge flow with an increase in the open sieve surface towards the edge zone.
Metal- or ceramic-based materials have a sufficient resistance against the existing thermal and chemical stresses, whereby metallic materials, especially aluminum-containing or -treated iron or steel alloys, are preferred in view of a reduction of the combustion chamber oscillations.
In a supplementary, especially preferred embodiment, the net-like structure is provided with a catalytically effective surface for supporting combustion.
It was shown that the thermoacoustic behavior of the burner can be positively changed with a net-like structure extending over the entire flow cross-section, and that as a result of this change a reduction of the oscillation tendency of the burner and therefore a stabilization of the flame are found. This positive effect apparently can be attributed to the nature of the net structure, i.e. the insulation and dampening of the sound waves propagating inside the combustion chamber. Pressure waves from the combustion chamber in the direction of the gas injection are partially dampened and partially insulated by the structure, i.e. are let through at a changed frequency or are reflected again. The essentially shorter propagation length of the sound waves reflected back into the combustion chamber causes the excitation of pressure oscillations in the combustion chamber to be shifted towards other frequency ranges and to be reduced.
The dampening and insulation behavior of the net structure is influenced by its material and dimensions.
This stabilizing effect of the net structure is additionally reinforced by the oxidation-promoting, catalytic action of its surface.
The mentioned effects result in synergies, which ultimately shift the extinction limit towards lower flame temperatures and therefore, at the same combustion temperature, improve the flame stability by increasing the interval between flame temperature and extinction limit temperature.
Another feature of the invention that should be emphasized is that existing burners can be retrofitted with little expenditure.
The invention can be used in premix burners known to the expert as such from the previously cited state of the art. The invention can be easily combined with all burner types disclosed in the cited publications and further developed from these publications and known per se to the expert; in view of the many possible forms of embodiments, these burner types are only incompletely reflected in the preferred embodiments mentioned in the secondary claims.